1
\$\begingroup\$

I am working with a LM2576T-ADJ that is taking as an input voltage a 2 cell Li-Po battery that in my application won't decrease below 8V and should output a voltage of approximately 6V to power a pair of Pololu Micrometal Gear Motors that will have a peak combined load of 3A.

I have followed the datasheet (https://www.ti.com/lit/ds/symlink/lm2576.pdf) and TI's Simple Switcher document (https://www.ti.com/lit/an/snva054c/snva054c.pdf?ts=1684356810650) to the best of my knowledge however when I assembled the circuit I got an output voltage of approximately 7.6V. My reading was taken with a multimeter as I don't have access to an oscilloscope and I am sure that the reading is somewhat correct as when I measure the voltage across the battery's leads I get 8V which corresponds to the voltage reading of a small Li-Po undervoltage module I have.

Below I have attached the manufacturer's schematic of the switching voltage regulator; the manufacturer's example layout for a PCB for the switching voltage regulator; my schematic for the switching voltage regulator circuit used in my project; and the layout of the components on my PCB.

enter image description hereenter image description hereenter image description hereenter image description here

In addition, the links to the components are: L1 D1 U17

Also this is a 2 layer PCB so the bottom layer (blue) is the GND layer that is a copper pour. I have identified the fact that the pins on the LM2576T-ADJ TO-220-5 are very close together which makes soldering hard and there might be a short however I did a continuity test using my multimeter and there were only small resistances between pins 3 and 5 which are both connected to ground.

Any help would be greatly appreciated as I want to get to the root cause of this issue as many projects I want to do in the future require high efficiency power supplies and I don't want to spend a lot of money on breakout boards with SMPS from the likes of Pololu.

\$\endgroup\$
9
  • \$\begingroup\$ The batteries won't go below 8 V? That's really low utilization for a 2-cell battery. Anyway, what's the voltage on the feedback pin look like? This looks like there's a failure of the feedback network, perhaps a resistor not soldered properly or something. \$\endgroup\$
    – Hearth
    Commented Jun 4, 2023 at 17:21
  • \$\begingroup\$ At the junction between R6 and R5 its 1.55V and at pin 4 it's 1.23V which is the Vref for the Feedback loop (does this have any correlation?). Also I don't want to go below 8V because I fear that the battery will lose too much charge - is this too conservative? \$\endgroup\$
    – DanielR
    Commented Jun 4, 2023 at 17:34
  • \$\begingroup\$ The fact that the voltage is that different is definitely a problem, you should measure the same voltage at both of those points. And li-ion batteries are usually used down to a charge of about 3.0 to 3.2 volts per cell (sometimes further below that even, but you start damaging the cell going too low); at 4.0 V per cell, the battery is still going to be around 95% charge. \$\endgroup\$
    – Hearth
    Commented Jun 4, 2023 at 17:37
  • \$\begingroup\$ These points being the the Feedback pin and the junction between the 2 resistors? \$\endgroup\$
    – DanielR
    Commented Jun 4, 2023 at 17:41
  • \$\begingroup\$ Those are the same circuit node, so they should have the same voltage. The fact that they don't indicates that something's not connected right somewhere. \$\endgroup\$
    – Hearth
    Commented Jun 4, 2023 at 17:45

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.